Abstract: A flap valve, designed to be installed and maintained in the standard cleanout wye of a conventional plastic plumbing system through the side channel thereof without removing said wye from service, comprising a flap with a flexible sealing gasket and a rigid backing plate, a valve seat with an O-ring seal and a locking tooth, which seat seals to the upstream channel of said wye when pressed into said channel, a hinge formed by attaching an extension of said sealing gasket to a curved downstream edge of said valve seat with a hinge clamp so that the extension of the sealing gasket will bend around said curved downstream edge, and a stop to prevent said flap from opening too widely, said stop being an extension of said hinge clamp.